Using C57BL/6J mice, nigrostriatal interactions during aging will be studied. The effects of age on axonal flow in the nigrostriated dopaminergic pathway will be measured and the numbers of neurones in the substantia nira and caudateputamen will be counted. The dynamics of estradiol (E2) receptors in the hypothalamus and pituitary will be studied in relation to plasma E2 during the estrous cycle and after castration. These studies will probe the critical linkage between hormones and genomic function in major neuroendocrine control centers.